In order to avoid that the cables, or rather the fiber connectors themselves, are damaged due to such incident radiation that are falling outside the core of the fiber, or due to radiation that are reflected back to the fiber connectors, methods are previously known to take care of such power loss. Specifically, it is previously known to have such incident radiation absorbed in a flowing coolant.
Optical fibre cables for transmitting high optical power are frequently used in industrial applications. Specifically they are used in cutting and welding operations by means of high-power laser radiation, but also in other industrial applications such as heating, detection or working operations in high-temperature environments this type of optical fiber cables can be used. By means of the optical fibers it is possible to design flexible manufacturing systems for transmitting radiation from the high power laser source to the workpiece. Laser sources which can be used in this context have average power from a few hundred watts up to several kilowatts.
Normally, an optical fiber has an inner core of glass for transmitting the radiation and one or more surrounding layers for optically “locking” the radiation into the core. Such a surrounding layer or layers are called the “cladding” of the fiber. Outside the cladding there are also one or more protecting layers to stabilize the fiber mechanically. These layers are called buffers or jackets.
When the fiber is built into a connector the fiber also has to be maintained in a correct position by some mechanical component.
When designing fiber systems for such high power radiation it is important to take care of radiation that falls outside the core of the fiber, due to for instance reflections against the workpiece or due to an incorrect focusing to the fiber, and cool it down in order to prevent uncontrolled heating in the system.
The main reasons for damages on the fiber connectors are                damages due to radiation that falls into the cladding and a subsequent leakage of radiation into the surrounding material, specifically into the area where the buffer and jacket are connected to the fiber, and        damages due to radiation impinging upon the mechanical detail that are adapted to hold the fiber.        
Different methods to take care of such unwanted power radiation and protect the fiber components are already known. One example is disclosed in DE 4305313, in which the radiation that falls into the cladding of the fiber is spread in a so-called mode stripper and absorbed by a metal surface. This surface can then be cooled from the outside of the device. Similar methods are described in EP 0 151 909 B1 and U.S. Pat. No. 4,575,181. In EP 0 349 312 B1 it is also described a method to mechanically grip the fiber which reduces the risk for any damages to the surrounding material to a minimum.
An optical fiber in which at least one of the end surfaces of the fiber core is provided with a rod made of a transparent material, for instance quartz, and having a larger diameter than the core diameter, is described in EP 0 619 508. At this end the fiber is provided with a reflector designed to conduct rays entering outside the fiber towards an area where they can be absorbed without causing any damage. In the illustrated embodiment this area is surrounded by a heat-abducting device with cooling fins, but it is also mentioned that water cooling means may be included in this area for cooling off the generated heat. Also in this case the cooling is provided from the outside of the device. A similar arrangement in which the end part of the fiber is provided with a hollow rod and a reflector is described in GB 2 255 199.
One example of a fiber connector which is used today is based on said EP 0 619 508. In this case the fiber is in optical contact with a rod made of quartz and the volume behind the quartz rod is swept over by a flowing coolant that absorbs the radiation, see more in detail in the accompanying FIG. 1. The coolant is normally water, which has an absorption depth of approximately 5 cm for radiation in the area of 1 micrometer.
In SE 509 706 and RU 2031420 it is described a method to take care of power loss in which the radiation heat completely or partially is absorbed directly in a flowing coolant instead of a metal. In SE 509 706 at least one of the contact ends of the fibre is located in a cavity filled with a flowing coolant so that radiation falling outside the fiber is entered into and absorbed at least partially by the coolant. According to a preferred embodiment the fiber is directly in contact with the surrounding coolant, for example water. The advantage by having the radiation absorbed directly in the coolant is a more effective cooling as no heat conducting through for instance a metal part is required before the heat is cooled off.
Due to the all increasing laser power used today the cooling capacity requirements have also been increased. There are two main disadvantages with the above-mentioned quartz rod design. In case of an incorrect focusing of the fiber connector the power density on the surface of the quartz rod might be too high and there is a risk for shock boiling effects in the flowing coolant resulting in damages to the connector. The risk for such damages are also increased due to the fact that the area adjacent to the surface of the quartz rod is not optimized from a flowing point of view. In order to avoid that radiation hits the connector body it is desired that the radiation is deflected towards the optical axis of the fiber.